Re: [PATCH RFC] svcrdma: Ignore source port when computing DRC hash

[Chuck Lever <chuck.lever@oracle.com>]

> On Jun 10, 2019, at 6:13 PM, Tom Talpey <tom@talpey.com> wrote:
> 
> On 6/10/2019 5:57 PM, Chuck Lever wrote:
>>> On Jun 10, 2019, at 3:14 PM, Tom Talpey <tom@talpey.com> wrote:
>>> 
>>> On 6/10/2019 1:50 PM, Chuck Lever wrote:
>>>> Hi Tom-
>>>>> On Jun 10, 2019, at 10:50 AM, Tom Talpey <tom@talpey.com> wrote:
>>>>> 
>>>>> On 6/5/2019 1:25 PM, Chuck Lever wrote:
>>>>>> Hi Tom-
>>>>>>> On Jun 5, 2019, at 12:43 PM, Tom Talpey <tom@talpey.com> wrote:
>>>>>>> 
>>>>>>> On 6/5/2019 8:15 AM, Chuck Lever wrote:
>>>>>>>> The DRC is not working at all after an RPC/RDMA transport reconnect.
>>>>>>>> The problem is that the new connection uses a different source port,
>>>>>>>> which defeats DRC hash.
>>>>>>>> 
>>>>>>>> An NFS/RDMA client's source port is meaningless for RDMA transports.
>>>>>>>> The transport layer typically sets the source port value on the
>>>>>>>> connection to a random ephemeral port. The server already ignores it
>>>>>>>> for the "secure port" check. See commit 16e4d93f6de7 ("NFSD: Ignore
>>>>>>>> client's source port on RDMA transports").
>>>>>>> 
>>>>>>> Where does the entropy come from, then, for the server to not
>>>>>>> match other requests from other mount points on this same client?
>>>>>> The first ~200 bytes of each RPC Call message.
>>>>>> [ Note that this has some fun ramifications for calls with small
>>>>>> RPC headers that use Read chunks. ]
>>>>> 
>>>>> Ok, good to know. I forgot that the Linux server implemented this.
>>>>> I have some concerns abot it, honestly, and it's important to remember
>>>>> that it's not the same on all servers. But for the problem you're
>>>>> fixing, it's ok I guess and certainly better than today. Still, the
>>>>> errors are goingto be completely silent, and can lead to data being
>>>>> corrupted. Well, welcome to the world of NFSv3.
>>>> I don't see another option.
>>>> Some regard this checksum as more robust than using the client's
>>>> IP source port. After all, the same argument can be made that
>>>> the server cannot depend on clients to reuse their source port.
>>>> That is simply a convention that many clients adopted before
>>>> servers used a stronger DRC hash mechanism.
>>>>>>> And since RDMA is capable of
>>>>>>> such high IOPS, the likelihood seems rather high.
>>>>>> Only when the server's durable storage is slow enough to cause
>>>>>> some RPC requests to have extremely high latency.
>>>>>> And, most clients use an atomic counter for their XIDs, so they
>>>>>> are also likely to wrap that counter over some long-pending RPC
>>>>>> request.
>>>>>> The only real answer here is NFSv4 sessions.
>>>>>>> Missing the cache
>>>>>>> might actually be safer than hitting, in this case.
>>>>>> Remember that _any_ retransmit on RPC/RDMA requires a fresh
>>>>>> connection, that includes NFSv3, to reset credit accounting
>>>>>> due to the lost half of the RPC Call/Reply pair.
>>>>>> I can very quickly reproduce bad (non-deterministic) behavior
>>>>>> by running a software build on an NFSv3 on RDMA mount point
>>>>>> with disconnect injection. If the DRC issue is addressed, the
>>>>>> software build runs to completion.
>>>>> 
>>>>> Ok, good. But I have a better test.
>>>>> 
>>>>> In the Connectathon suite, there's a "Special" test called "nfsidem".
>>>>> I wrote this test in, like, 1989 so I remember it :-)
>>>>> 
>>>>> This test performs all the non-idempotent NFv3 operations in a loop,
>>>>> and each loop element depends on the previous one, so if there's
>>>>> any failure, the test imemdiately bombs.
>>>>> 
>>>>> Nobody seems to understand it, usually when it gets run people will
>>>>> run it without injecting errors, and it "passes" so they decide
>>>>> everything is ok.
>>>>> 
>>>>> So my suggestion is to run your flakeway packet-drop harness while
>>>>> running nfsidem in a huge loop (nfsidem 10000). The test is slow,
>>>>> owing to the expensive operations it performs, so you'll need to
>>>>> run it for a long time.
>>>>> 
>>>>> You'll almost definitely get a failure or two, since the NFSv3
>>>>> protocol is flawed by design. But you can compare the behaviors,
>>>>> and even compute a likelihood. I'd love to see some actual numbers.
>>>> I configured the client to disconnect after 23711 RPCs have completed.
>>>> (I can re-run these with more frequent disconnects if you think that
>>>> would be useful).
>>>> Here's a run with the DRC modification:
>>>> [cel@manet ~]$ sudo mount -o vers=3,proto=rdma,sec=sys klimt.ib:/export/tmp /mnt
>>>> [cel@manet ~]$ (cd /mnt; ~/src/cthon04/special/nfsidem 100000)
>>>> testing 100000 idempotencies in directory "./TEST"
>>>> [cel@manet ~]$ sudo umount /mnt
>>>> Here's a run with the stock v5.1 Linux server:
>>>> [cel@manet ~]$ sudo mount -o vers=3,proto=rdma,sec=sys klimt.ib:/export/tmp /mnt
>>>> [cel@manet ~]$ (cd /mnt; ~/src/cthon04/special/nfsidem 100000)
>>>> testing 100000 idempotencies in directory "./TEST"
>>>> [cel@manet ~]$
>>>> This test reported no errors in either case. We can see that the
>>>> disconnects did trigger retransmits:
>>>> RPC statistics:
>>>>   1888819 RPC requests sent, 1888581 RPC replies received (0 XIDs not found)
>>>>   average backlog queue length: 119
>>> 
>>> Ok, well, that's 1.2% error rate, which IMO could be cranked up much
>>> higher for testing purposes. I'd also be sure the server was serving
>>> other workloads during the same time, putting at least some pressure
>>> on the DRC. The op rate of a single nfsidem test is pretty low so I
>>> doubt it's ever evicting anything.
>>> 
>>> Ideally, it would be best to
>>> 1) increase the error probability
>>> 2) run several concurrent nfsidem tests, on different connections
>>> 3) apply some other load to the server, e.g. several cthon basics
>>> 
>>> The idea being to actually get the needle off of zero and measure some
>>> kind of difference. Otherwise it really isn't giving any information
>>> apart from a slight didn't-break-it confidence. Honestly, I'm surprised
>>> you couldn't evince a failure from stock. On paper, these results don't
>>> actually tell us the patch is doing anything.
>> I boosted the disconnect injection rate to once every 11353 RPCs,
>> and mounted a second share with "nosharecache", running nfsidem on
>> both mounts. Both mounts are subject to disconnect injection.
>> With the current v5.2-rc Linux server, both nfsidem jobs fail within
>> 30 seconds.
>> With my DRC fix applied, both jobs run to completion with no errors.
>> It takes more than an hour.
> 
> Great, that definitely proves it. It's relatively low risk, and
> should go in.
> 
> I think it's worth thinking through the best way to address this
> across multiple transports, especially RDMA, where the port and
> address behaviors may differ from TCP/UDP. Perhaps tossing the whole
> idea of using the 5-tuple should simply be set aside, after all
> these years. Even though NFSv4.1 already has!
> 
> Thanks for humoring me with the extra testing, by the way :-)

Sho 'nuf. Worth it to ensure we are all on the same page now.

I'll send this to Bruce with some clarifications to the description.


> Tom.
> 
>> Here are the op metrics for one of the mounts during a run that
>> completes successfully:
>> RPC statistics:
>>   4091380 RPC requests sent, 4088143 RPC replies received (0 XIDs not found)
>>   average backlog queue length: 1800
>> ACCESS:
>>        	300395 ops (7%)         301 retrans (0%)        0 major timeouts
>>         avg bytes sent per op: 132	avg bytes received per op: 120
>>         backlog wait: 4.289199  RTT: 0.019821   total execute time: 4.315092 (milliseconds)
>> REMOVE:
>>        	300390 ops (7%)         168 retrans (0%)        0 major timeouts
>>         avg bytes sent per op: 136	avg bytes received per op: 144
>>         backlog wait: 2.368664  RTT: 0.070106   total execute time: 2.445148 (milliseconds)
>> MKDIR:
>>       	200262 ops (4%)         193 retrans (0%)        0 major timeouts
>>         avg bytes sent per op: 158	avg bytes received per op: 271
>>         backlog wait: 4.207034  RTT: 0.075421   total execute time: 4.289101 (milliseconds)
>> RMDIR:
>>       	200262 ops (4%)         100 retrans (0%)        0 major timeouts
>>         avg bytes sent per op: 130	avg bytes received per op: 144
>>         backlog wait: 2.050749  RTT: 0.071676   total execute time: 2.128801 (milliseconds)
>> LOOKUP:
>>        	194664 ops (4%)         233 retrans (0%)        0 major timeouts
>>         avg bytes sent per op: 136	avg bytes received per op: 176
>>         backlog wait: 5.365984  RTT: 0.020615   total execute time: 5.392769 (milliseconds)
>> SETATTR:
>>        	100130 ops (2%)         86 retrans (0%)         0 major timeouts
>>         avg bytes sent per op: 160	avg bytes received per op: 144
>>         backlog wait: 3.520603  RTT: 0.066863   total execute time: 3.594327 (milliseconds)
>> WRITE:
>>        	100130 ops (2%)         82 retrans (0%)         0 major timeouts
>>         avg bytes sent per op: 180	avg bytes received per op: 136
>>         backlog wait: 3.331249  RTT: 0.118316   total execute time: 3.459563 (milliseconds)
>> CREATE:
>>        	100130 ops (2%)         95 retrans (0%)         0 major timeouts
>>         avg bytes sent per op: 168	avg bytes received per op: 272
>>         backlog wait: 4.099451  RTT: 0.071437   total execute time: 4.177479 (milliseconds)
>> SYMLINK:
>>        	100130 ops (2%)         83 retrans (0%)         0 major timeouts
>>         avg bytes sent per op: 188	avg bytes received per op: 271
>>         backlog wait: 3.727704  RTT: 0.073534   total execute time: 3.807700 (milliseconds)
>> RENAME:
>>        	100130 ops (2%)         68 retrans (0%)         0 major timeouts
>>         avg bytes sent per op: 180	avg bytes received per op: 260
>>         backlog wait: 2.659982  RTT: 0.070518   total execute time: 2.738979 (milliseconds)
>> LINK:
>> 	100130 ops (2%) 	85 retrans (0%) 	0 major timeouts
>> 	avg bytes sent per op: 172	avg bytes received per op: 232
>> 	backlog wait: 3.676680 	RTT: 0.066773 	total execute time: 3.749785 (milliseconds)
>> GETATTR:
>> 	230 ops (0%) 	81 retrans (35%) 	0 major timeouts
>> 	avg bytes sent per op: 170	avg bytes received per op: 112
>> 	backlog wait: 1584.026087 	RTT: 0.043478 	total execute time: 1584.082609 (milliseconds)
>> READDIRPLUS:
>> 	10 ops (0%)
>> 	avg bytes sent per op: 149	avg bytes received per op: 1726
>> 	backlog wait: 0.000000 	RTT: 0.300000 	total execute time: 0.400000 (milliseconds)
>> FSINFO:
>> 	2 ops (0%)
>> 	avg bytes sent per op: 112	avg bytes received per op: 80
>> 	backlog wait: 0.000000 	RTT: 0.000000 	total execute time: 0.000000 (milliseconds)
>> NULL:
>> 	1 ops (0%)
>> 	avg bytes sent per op: 40	avg bytes received per op: 24
>> 	backlog wait: 2.000000 	RTT: 0.000000 	total execute time: 3.000000 (milliseconds)
>> READLINK:
>> 	1 ops (0%)
>> 	avg bytes sent per op: 128	avg bytes received per op: 1140
>> 	backlog wait: 0.000000 	RTT: 0.000000 	total execute time: 0.000000 (milliseconds)
>> PATHCONF:
>> 	1 ops (0%)
>> 	avg bytes sent per op: 112	avg bytes received per op: 56
>> 	backlog wait: 0.000000 	RTT: 0.000000 	total execute time: 0.000000 (milliseconds)
>>> Tom.
>>> 
>>>> ACCESS:
>>>>         300001 ops (15%)        44 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 132	avg bytes received per op: 120
>>>>         backlog wait: 0.591118  RTT: 0.017463   total execute time: 0.614795 (milliseconds)
>>>> REMOVE:
>>>>        	300000 ops (15%)        40 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 136	avg bytes received per op: 144
>>>>         backlog wait: 0.531667  RTT: 0.018973   total execute time: 0.556927 (milliseconds)
>>>> MKDIR:
>>>>      	200000 ops (10%)        26 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 158      avg bytes received per op: 272
>>>>         backlog wait: 0.518940  RTT: 0.019755   total execute time: 0.545230 (milliseconds)
>>>> RMDIR:
>>>> 	200000 ops (10%)        24 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 130	avg bytes received per op: 144
>>>>         backlog wait: 0.512320  RTT: 0.018580   total execute time: 0.537095 (milliseconds)
>>>> LOOKUP:
>>>>        	188533 ops (9%)         21 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 136	avg bytes received per op: 174
>>>>         backlog wait: 0.455925  RTT: 0.017721   total execute time: 0.480011 (milliseconds)
>>>> SETATTR:
>>>>         100000 ops (5%)         11 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 160	avg bytes received per op: 144
>>>>         backlog wait: 0.371960  RTT: 0.019470   total execute time: 0.398330 (milliseconds)
>>>> WRITE:
>>>>       	100000 ops (5%)         9 retrans (0%)  0 major timeouts
>>>>         avg bytes sent per op: 180	avg bytes received per op: 136
>>>>         backlog wait: 0.399190  RTT: 0.022860   total execute time: 0.436610 (milliseconds)
>>>> CREATE:
>>>>        	100000 ops (5%)         9 retrans (0%)  0 major timeouts
>>>>         avg bytes sent per op: 168	avg bytes received per op: 272
>>>>         backlog wait: 0.365290  RTT: 0.019560   total execute time: 0.391140 (milliseconds)
>>>> SYMLINK:
>>>>      	100000 ops (5%)         18 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 188	avg bytes received per op: 272
>>>>         backlog wait: 0.750470  RTT: 0.020150   total execute time: 0.786410 (milliseconds)
>>>> RENAME:
>>>>      	100000 ops (5%)         14 retrans (0%)         0 major timeouts
>>>>         avg bytes sent per op: 180	avg bytes received per op: 260
>>>>         backlog wait: 0.461650  RTT: 0.020710   total execute time: 0.489670 (milliseconds)
>>>> --
>>>> Chuck Lever
>> --
>> Chuck Lever

--
Chuck Lever